The laboratory has been engaged for a number of years in pursuing the elucidation of factors which regulate liver cell growth, both normal and malignant. This laboratory first described Hepatic Stimulator Substance (HSS, a 7500 dalton protein which stimulates normal and malignant liver cell growth, in vivo and in vitro. It is organ specific but species non- specific. HSS has now been purified to homogeneity, with a greater than 100,000 fold increase in specific activity. The present proposal consists of three parts: 1. Molecular characterization of HSS. Standard molecular biology techniques will be utilized to sequence HSS and construct a DNA library from regenerating and/or weanling rat liver from which to clone the gene for HSS. Synthetic partial sequences of HSS will be prepared and the gene for HSS expressed in E coli for production of large quantities of HSS. 2. Development of highly specific and sensitive assays of HSS. Polyclonal antibodies against HSS will be raised in female leghorn chickens. Monoclonal antibodies will be prepared against partial synthetic sequences of HSS conjugated to keyhole limpet hemocyanin carrier protein, HSS specific cDNA probes will also be constructed for studies of HSS gene regulation. 3. Production of adequate amounts of expressed recombinant and/or synthetic HSS for physiological studies, will also be pursued, with initial efforts to express HSS in single E coli expression systems. Availability of a truly organ specific growth promoter will allow investigation of differences in growth control of normal and malignant hepatocytes. When pure HSS is available in adequate quantities, it may also prove beneficial in clinical situations where rapid liver regeneration is required for survival, eg fulminant hepatitis, hepatic trauma and liver transplantation of split (reduced size) organs.